Multilayer wiring boards in which insulating layers and conductive layers (wiring layers) are layered alternately have been known as wiring boards for mounting semiconductor elements such as semiconductor integrated circuit elements, for example. In such a multilayer wiring board, a via hole or a through-hole extending so as to pass through one or more insulating layers in the thickness direction thereof is formed to electrically connect one conductive layer to another conductive layer.
In a manufacturing process of the multilayer wiring board, a wiring board material, which is formed by an insulating layer and a conductive layer layered on each other, is subjected to drill machining or laser machining to remove part of the insulating layer, thereby forming a through hole such as a via hole or a through-hole. When such a through hole is formed in the insulating layer, smear (residue), resulting from the material constituting the insulating layer, is generated in the wiring board material. Thus, a desmear treatment for removing such smear is performed on the wiring board material.
A wet treatment method of treating smear with an alkaline solution prepared by dissolving potassium permanganate or sodium hydroxide and a dry treatment method of treating smear with ultraviolet rays and ozone or oxygen radicals generated by such ultraviolet rays have been known in the art as the desmear treatments. In such a desmear treatment method, the smear attached to the interior of the via hole or the through-hole is removed and a surface of the insulating layer is roughened.
In the above-described desmear treatment method, however, if the desmear treatment is performed under conditions capable of sufficiently removing the smear, the surface of the insulating layer is excessively roughened. This is because the filler contained in the insulating layer is exposed on the surface of the insulating layer due to the excessive decomposition of the resin constituting the insulating layer. Thus, if the line and space of the conductive layer formed on the insulating layer are each not more than 15 μm, for example, problems such as the fall of wiring and deterioration in responsiveness for high-frequency signals occur.
In order to solve such problems, methods according to which in a state where a protective layer is formed on a surface of an insulating layer, a through hole passing through the protective layer and the insulating layer is formed, and then a desmear treatment is performed on the inner surface of the through hole (see Patent Literatures 1 and 2) have been proposed.
In such a method, however, the protective layer needs to be removed after the desmear treatment. Moreover, if the surface roughness of the insulating layer is too low, the surface of the insulating layer further needs to be roughened. Thus, a problem of complicating the entire treatment process occurs.